1. Field of the Invention
This invention relates to projection screens and more particularly, to semi-specular front projection screens for use in visual display systems.
2. Description of the Prior Art
When the projector and an observer of a projected image are located on the same side of a projecting screen, this screen is commonly referred to as a front projection screen. Front projection screens can be classified according to their light reflecting properties, as lambertian, reflex-reflective or semi-specular.
The lambertian screen is coated with a light diffusing material such as a matte white paint. The screen has a generally uniform brightness for all viewing angles and a gain of about one.
Reflex-reflective screens are constructed so that a beam of light incident on the front of the screen is reflected, as a cone of light, back towards the light source, even though the incident beam strikes the screen at an angle. The commonly used, glass beaded screen exhibits such selective light return in the direction of incidence. A glass beaded screen is made of glass beads bonded to a white surface and generally exhibits a gain of 2-3.
As its name implies, the reflection characteristics of a semi-specular screen resemble those of a mirror. The peak luminance with such a screen is in the direction of specular reflectivity. The more specular the screen, the higher will be the gain and directionality of the reflected light.
There are many factors which influence the selection of a projection screen including minimum resolving power, minimum acceptable brightness, and minimum viewing field. Screen selection is often a compromise in which a particular advantage is chosen in spite of accompanying disadvantages. Probably the most important factor in screen selection is the minimum acceptable brightness of the projected image.
Image brightness is, generally, a function of a number of variables including the light output of the projector, the ambient light conditions, the screen gain and the relative locations of the projector, screen and observer. In many cases, the locations of the projector, screen and observer are predetermined and the light flux from the projector is fixed. In these cases, images brightness varies directly as a function of screen gain and ambient light conditions. If the screen gain is restricted to relatively low levels, as it is with most matte white and glass beaded screens, then required image brightness can usually only be obtained in a darkened room. If the gain of the screen can be increased, then the severe restrictions on ambient lighting could be eased.
Theoretically, a mirror (i.e., a purely specular screen) would provide maximum gain and minimum limitations on ambient light conditions. However, a truely specular screen would suffer from severe "fireballing", i.e., the viewer would see an image of the projected light source rather than the projected image.
Semi-specular screens have been designed to provide greater gains with minimal fireballing. A regular surface painted with a metallic paint will function as a semi-specular screen. Another semi-specular screen, employing a multitude of tiny prismatic optical elements mounted on a backing surface, is described in a paper entitled "Bright-Screen Projection Systems for Data Display", by Allan R. Fultz, presented at the 5th Annual Technical Symposium of the Society of Photo-optical Instrumentation Engineers, Aug. 4, 1960.
This Fultz paper (Library of Congress Catalog No. TR 16.6 F 959b) contains an excellent discussion of the various optical properties of the different types of front projection screens. It also teaches a host of applications for semi-specular screens. These applications include flight simulation displays, radar displays, airborne visual displays and multiple projector/observer systems. The Fultz paper is incorporated by reference herein to illustrate the state-of-the-art and to indicate the many uses for a semi-specular screen such as that of the present invention.